The present invention relates generally to the field of wavelength division multiplexing within optical transmission systems, and more particularly to the field of wavelength division multiplexing using band separation within a generic spectral emission range of a rare-earth-doped fiber amplifier.
In optical transmission systems, optical fibers doped with rare-earth elements such as erbium provide a useful component for amplifying signals passing across a long distance link. These fiber amplifiers, when pumped with a first characteristic wavelength, provide gain to a transmission signal at a second characteristic wavelength. When erbium is used as the rare-earth dopant, the pump wavelength typically is either 980 nm or 1480 nm, which results in a stimulated emissionspectrum for the amplifier across a band of about 1528-1562 nm. Therefore, the erbium-doped fiber amplifier will amplify transmission signals passing through it at these wavelengths.
An optical transmission system using erbium-doped fiber amplifiers, however, suffers from several limitations due to the emission characteristics of the amplifier across the wavelength range. For one, the spectral emission of the erbium fiber is non-flat across the wavelength band of 1528-1562. As a result, only a narrow band of wavelengths have conventionally been used to obtain equivalent gain across the band. Many systems have chosen 1550 nm and its surrounding wavelengths as the narrow band due to the relatively flat response of the erbium-doped fiber amplifier in this region. When a high number of channels using dense wavelength division multiplexers (WDM) are applied to the erbium-doped fiber amplifier, techniques such as equalizing means must be employed in an attempt to flatten the gain of the amplifier across the bandwidth of the system. These equalizing means encumber system design. In addition, the cascading of amplifiers in a large WDM system compounds the issues with non-flat gain and imposes further fundamental limitations on system design.
FIG. 1 is a graph of a generalized spectral emission range of 1528-1562 nm for an erbium-doped fiber amplifier showing the different gain for channels of signals traveling through an optical communication link including the erbium-doped fiber. As shown in FIG. 1, the gain in a lower region between 1528 nm and 1541 nm is non-flat, whereas the gain in the higher region is mostly flat. In WDM systems, discrete wavelengths within a small tolerance, otherwise known as channels, are used to carry modulated information. For channels in the lower region, the disparity in gain for signals passing through an erbium-doped fiber amplifier may cause unequal amplification among the channels. The disparity becomes more significant when the channels pass though a cascade of amplifiers that have similar gain characteristics. The differences in gain among the channels can become extreme enough to cause channels with very low gain to fall below a predetermined noise cutoff level. The performance specifications of a receiver positioned downstream from the amplifiers may dictate the noise cutoff level. Channels falling below the noise cutoff are not detected, or detected poorly, effectively eliminating those channels or reducing their reliability.
To overcome the gain disparity problems, optical transmission systems have used equalizing devices such as notch filters as a dual core fiber, interferential filters, long period grating, chirped gratings, or hybrid active fiber, for example, to flatten the gain characteristic. Some of these techniques are discussed in U.S. Pat. No. 5,260,823. However, these equalizing devices are only effective in limited applications, such as linear conditions, and are thus liable to maintain continued gain disparities when applied to the erbium-doped fiber spectral emission range of 1528-1562 nm. Thus, due to the potentially large gain disparities in the lower channel region and the corresponding problems of flattening the gain characteristics, optical transmission systems have been limited to using the higher end of the erbium-doped fiber amplifier spectral emission range.
U.S. Pat. No. 5,392,154 proposes a self-regulating multiwavelength optical amplifier module providing desired channel-by-channel power regulation and immunity to transient interchannel cross-saturation. The proposed amplifier module includes a plurality of pump-shared parallel fiber amplifiers operated in gain-saturation and connected between a demultiplexer and a multiplexer. Each of the fiber amplifiers individually amplifies one channel at a single wavelength. An optional first gain stage comprising a strongly pumped erbium-doped fiber amplifier improves performance with higher optical signal-to-noise ratio.
E.P. Patent Application No. 445,364A proposes an optical fiber communication system providing a connection between a central station and a number of subscriber stations and including an optical amplifier (OV) having wavelength selective couplers at input and output adapted to direct a first wavelength .lambda.1 into the amplifier and a second wavelength .lambda.2 into a bridging conductor (U).
U.S. Pat. No. 5,452,116 discloses a wavelength division multiplexed optical transmission system incorporating a concatenation of optical amplifiers. The multiplexed signal passes through a limited number of amplifiers in which all channels are amplified together. Then, the signal is demultiplexed and the individual channels are separately amplified and then remultiplexed together. In instances where a set of channels may be grouped into subsets of channels for which the individual channel spacing is so close that any differential amplification is negligible, then the set may be amplified separately from another set.
Similarly, U.S. Pat. No. 5,608,571 discloses an optical amplifier for a WDM system that has a set of optically amplifying fibers arranged with an associated spectrally selective Bragg reflector. Different spectral components of an input signal propagate through different ones of the optically amplifying fibers based on the reflection band of the associated Bragg reflectors and return to a transmission path.
U.S. Pat. No. 5,563,733 discloses an apparatus for optically amplifying a plurality of signals having different wavelengths where a first signal among the plurality propagates through a part of a series of rare-earth-doped optical fibers and a second signal among the plurality propagates through all of the series of rare-earth-doped optical fibers. The disclosed arrangement aims to provide an equalizing gain for signals having different input powers, for example a digital signal that has a small input power and an analog signal that has a large input power. A WDM coupler separates the analog signal before it passes through all of a series of fiber amplifiers.
Applicants have discovered that the limited region in the erbium-doped fiber amplifier spectral emission range used for transmitting signals does not fulfill the needs of dense WDM systems, particularly WDM systems having sixteen or more channels and using erbium-doped fiber amplifiers. Applicants have found that the prior arrangements of separating certain types of signals from a cascade of amplifiers or separately amplifying groups of channels having negligible differential amplification fall short of fulfilling the needs of dense WDM systems.
Applicants have observed that prior art approaches. may suffer from having the power of individual output channels not be independent from the other channels. Moreover, Applicants have observed that when band-separated equalizing techniques are employed within the stages of amplification in a WDM system, the power of the channels as well as the spectra can be effectively separated and made independent. In this way, relatively consistent output power between channels of a dense WDM system can be obtained, and the power performance of the channels of a sub-band can be made relatively independent of the presence or absence of channels in other sub-bands.